The use of inter-vehicular communication is an essential part of future smart cars and roads. For example, prior art document WO 01/01587 A2 discloses a dynamic wireless networking between vehicles, wherein each vehicle is capable of transmitting information and receiving information.
Generally, vehicle-to-vehicle messages can be exchanged in an ad hoc network by using a M[edium]AC[cess] protocol like IEEE802.11 that regulates the access to the shared communication medium without the usage of a central controller (cf. Ysuhiko Inoue, Masao Nagakawa, “MAC protocol for inter-vehicle communication network using spread spectrum technique”, Vehicle navigation & information conference proceedings (IEEE), 1994).
Furthermore, in inter-vehicular communication, groups of vehicles can be clustered, for example by choosing a multicast ID in the cluster. In this context, prior art document U.S. Pat. No. 6,397,149 B1 discloses a processional travel control apparatus by which respective vehicle groups travelling in processions can be distinguished.
As long as the relative speed between the cars is low, the relative positional change of the cars is low and clustering is a not too difficult task. To establish and maintain a cluster, cars must not have a too large distance and must be also within this maximum distance during a longer period; otherwise, too much communication overhead will be required to keep the cluster up-to-date of the latest cluster configuration.
One of the main problems is that if two clusters come into contact they can interfere with each other. For this reason, the clusters have to be separated using different channels. To this aim, prior art document U.S. Pat. No. 6,397,149 B1 proposes to assign the frequency band according to the vehicle group ID. To achieve communication between vehicles in the same lane without interfering with communication between vehicles in other lanes, one of the frequencies is switched to a different frequency when the vehicle groups approach each other and when the frequency bands of the vehicle groups have been the same.
Since the number of available channels is directly bound to the cost of the communication system, this number of available channels has to be kept as low as possible, so a re-use of the channels is required. In this context, the problem arises that clusters travelling in opposite directions will come into contact with each other quite often whereby the probability of the clusters using the same channel can be quite high (depending on the number of channels available).
Another common problem occurring when IEEE802.11 compliant equipment is used outdoors is the so-called “hidden node problem”. This problem occurs when a wireless node cannot hear one or more of the other wireless nodes, as a consequence of which the media access protocol cannot function properly. When this happens, multiple nodes will attempt to transmit their data over the shared medium simultaneously, causing signal interference with one another and consequently collisions on data transmission.
The hidden node problem gets an extra dimension when a C[lear]T[o]S[end] signal from a receiver might be valid at the moment the CTS is given but a short time later a car approaching the cluster might interfere the data transfer without being aware of it.
One of the main difficulties in vehicle-to-vehicle networks, in particular in car-to-car networks, is that the configuration of the network changes rapidly due to the velocity of the individual nodes especially if the vehicles have different directions. In connection therewith or independently thereof, another problem is the rapidly changing density of the cars.
In this context, it has to be taken into consideration that power control mechanisms, collision avoidance techniques and synchronization normally need some time to adapt to a new situation. If the available time is not sufficient, these techniques will fail and the network performance can collapse.
A potential solution for these problems might be found by setting up a separate channel for each peer-to-peer connection and for each cluster communication. However, this would make the receiver of a node quite expensive to support all its active channels in parallel. Moreover, the number of frequencies would have to be very high, in case the channels are separated by frequency.
Concerning the above-mentioned M[edium]AC[cess] layer of a protocol for car-to-car communication purpose, a lot of work has already been done. The problem of medium accessing becomes of primary importance because the number of nodes and their mobility can vary within a very large range as well as on a very large scale.
Some conventional solutions have been proposed that consider                T[ime]D[ivision]M[ultiple]A[ccess] (cf. Lechlan B. Michael, Masao Nakagawa, “Non-platoon inter-vehicle communication using multiple hops”, IEEE Trans. Commun. Vol. E82-B No. 10, October 1999) or        C[ode]D[ivision]M[ultiple]A[ccess] (cf. Ysuhiko Inoue, Masao Nagakawa, “MAC protocol for inter-vehicle communication network using spread spectrum technique”, Vehicle navigation & information conference proceedings (IEEE), 1994).        
In this context, CDMA seems to be the preferable solution because CDMA does not require any synchronization, which could be quite difficult to achieve in a decentralized and highly variable environment. However, in case the channels are separated by CDMA for setting up a separate channel for each peer-to-peer connection and each cluster communication the codes would have to be very long for differentiating the channels.
By using smart antenna arrays, it is possible to use S[pace]D[ivision]M[ultiple]A[ccess]. In this case, users may use the same frequency, time, or code allocations over the air interface and may only be separated spatially. This enables SDMA to be a complementary scheme to F[requency]D[ivision]M[ultiple]A[ccess], T[ime]D[ivision]M[ultiple]A[ccess], and C[ode]D[ivision]M[ultiple]A[ccess]; thus, S[pace]D[ivision]M[ultiple]A[ccess] provides increased capacity within congested areas (cf.
http://www.xilinx.com/publications/xcellonline/partners/xc_pdf/xc_nallatec h45.pdf). In this context, prior art document WO 02/41643 A2 discloses a method for operating a synchronous SDMA and a CDMA.
Anyway, cluster organization seems to bring significant advantages with respect to simple peer-to-peer connections. One of the main issues when considering clustering for car-to-car communication purpose is how to separate different clusters to avoid interference. Some work has already been done concerning ways to separate codes in car-to-car environment.
In this context, prior art document US 2002/0198632 A1 discloses a method and an arrangement for communicating between vehicles wherein a code depending on the area where the vehicles are located is assigned. This anyway seems to require a real large number of codes, and does not resolve the problem associated with the interference between clusters of cars travelling in opposite directions.
In the prior art article “Interference characteristics in inter-vehicle communication from oncoming vehicles” from Lachlan B. Michael and Masao Nakagawa, (Vehicular Technology Conference, Amsterdam, Netherlands, September 1999, volume 2, pages 753 to 757, ISBN 0-7803-5435-4), the use of directional or beam antennas, coupled with separating the frequency band into forward transmit/reverse receive operation, and reverse transmit/forward receive operation is proposed to solve the problems of interference from oncoming vehicles.
Since this known system is only based on forward and backward directions, it is feasible only to reduce interference between cars having opposite directions. Thus, the system presented in this article is not suitable when a vehicle is passing a cross-over or a car is entering a highway, because it is not possible to make a difference between cars having an other direction than forward or reverse; for example, this known system does not enable to make a difference between cars having west-to-east direction and cars having southwest-to-northeast direction.
Starting from the disadvantages and shortcomings as described above and taking the prior art as discussed into account, an object of the present invention is to provide a communication system as well as a vehicle comprising such a communication system and a communication method for vehicles moving in any different directions within the same area, wherein interference is to be eliminated.
The object of the present invention is achieved by a communication system comprising the features of claim 1 as well as by a method comprising the features of claim 5 and by a vehicle comprising the features of claim 11. Advantageous embodiments and expedient improvements of the present invention are disclosed in the respective dependent claims.
The present invention is principally based on the idea of direction dependent channel selection for ad hoc wireless network of vehicles wherein a pre-clustering of at least one network can be defined.